(1) Field of the Invention
The present invention relates to a communication network system and a method for synchronously controlling path connection. More particularly, the invention relates to a communication network system, an ATM node, and a method for synchronously controlling path connection for instantaneously controlling path connection at a predesignated time.
(2) Description of the Related Art
In an ATM (Asynchronous Transfer Mode) communication network, communications are conducted among terminals in a form that a plurality of communication paths (virtual paths or virtual connections) are multiplexed on a physical line connecting nodes.
In the ATM communication field, a technique of automatically switching a communication path in which a failure occurs to another communication path detouring the failure position is known. There is the upper limit in the transmission capability (bandwidth) of a connection line between nodes. When an idle bandwidth is narrow, the communication path cannot be established in a bandwidth desired by the user. Consequently, a technique of reserving a bandwidth while designating a communication start time in advance and automatically assuring the bandwidth at the reserved time is known.
For example, Japanese Unexamined Patent Application No. 7-154392 proposes a path bandwidth setting method. According to the method, a node which receives a bandwidth reservation from a user terminal issues a reservation request to a bandwidth managing unit, and the bandwidth managing unit determines whether the reservation is acceptable or not. When the reservation is acceptable, a reserved bandwidth of the path and reserved time are stored as bandwidth management information. At the reserved time, a bandwidth change is notified to nodes at both ends of the path.
In a bandwidth reservation control system proposed in Japanese Unexamined Patent Application No. 10-173662, a bandwidth reservation request in which a communication destination terminal, a bandwidth, scheduled communication start and end times, and a reservation class are designated is issued from a terminal to a network managing system. The reservation class includes a preferential reservation for immediately assuring a bandwidth and a normal reservation of attempting assurance of a bandwidth at the scheduled communication start time. When the bandwidth reservation request is received from the terminal, in the case of the normal reservation, the network managing system performs a reservation registering process and, at the scheduled communication start time, starts a route determining process capable of assuring the bandwidth. In the case of the preferential reservation, on receipt of the bandwidth reservation request, the route determining process is immediately performed. After determining a route having the user-designated bandwidth, a bandwidth assuring process is executed. The bandwidth assurance in this case means a state that, although a connection to an ATM switch has not be established, the bandwidth cannot be used by other request calls. The connection to the ATM switch is established at the scheduled communication start time.
In the conventional path bandwidth control for designating a communication start time (reserved time) and automatically setting a path, a network management system issues a control instruction for setting a path to nodes positioned at both ends of a reserved path or each of nodes on a reserved path at the reserved time. A delay in time occurs in a period from the issuance of the control instruction to completion of an actual path setting operation performed by each of the nodes. A plurality of nodes therefore cannot set paths instantaneously at the reserved time.
Assuming now that, for example, a digital TV relay network for connecting broadcasting stations in major cities in a country via an ATM network is constructed in order to relay a TV program produced by one of the stations to other stations via the ATM network, and to simultaneously broadcast the program by the plurality of stations, frequent switching the source stations are required in accordance with a scheduled TV program table in each broadcasting station. In this case, a plurality of nodes on the ATM network connecting the broadcasting stations have to instantaneously set or release paths at a predesignated time synchronously with each other.